The present invention relates to a process for deacidifying a fluid stream which comprises sour gases as impurities, and a wash liquid or absorption liquid for use in a process of this type.
In numerous chemical industry processes, fluid streams occur which comprise sour gases, for example CO2, H2S, SO2, CS2, HCN, COS or mercaptans, as impurities. These fluid streams can be, for example, gas streams (such as natural gas, synthesis gas from heavy oil or heavy residues, refinery gas, or reaction gases produced in the partial oxidation of organic materials, for example coal or oil), or liquid or liquefied hydrocarbon streams (such as liquefied petroleum gas (LPG) or natural gas liquid (NGL)).
Before these fluids can be transported or further processed, the sour gas content of the fluid must be significantly reduced. CO2, for example, must be removed from natural gas, since a high CO2 concentration reduces the heating value of the gas. In addition, CO2, together with water which is frequently entrained in the fluid streams, can lead to corrosion on pipes and fittings.
Removing sulfur compounds from these fluid streams is of special importance for a number of different reasons. For example, the sulfur compound content of natural gas must be reduced directly at the natural gas source by suitable treatment measures, since the sulfur compounds, in the water which is frequently entrained by the natural gas, form acids which have a corrosive action. To transport natural gas in a pipeline, therefore preset limits of the sulfur compound impurities must be complied with. Furthermore, numerous sulfur compounds are, even at low concentrations, foul-smelling and, especially hydrogen sulfide (H2S), toxic.
Therefore, numerous processes have previously been developed for removing sour gas constituents from fluid streams such as hydrocarbon gases, LPG or NGL. In the most widespread processes, the sour gas-containing fluid mixture is brought into contact with an organic solvent or an aqueous solution of an organic solvent in a gas scrubber or liquid/liquid extraction step.
Gas scrubbing processes and corresponding scrubbing solutions used in these processes are also covered by extensive patent literature. In principle, a distinction can be made between two different types of absorbants or solvents for gas scrubbing:
Firstly, physical solvents are used, in which, after absorption has been completed, the dissolved sour gases are present in molecular form. Typical physical solvents are cyclotetramethylene sulfone (sulfolane) and its derivatives, aliphatic acid amides, NMP (N-methylpyrrolidone), N-alkylated pyrrolidones and corresponding piperidones, methanol and mixtures of dialkyl ethers of polyethylene glycols (Selexol®, Union Carbide, Danbury, Conn., USA).
Secondly, chemical solvents are used whose mode of action is based on chemical reactions in which, after absorption is completed, the dissolved sour gases are present in the form of chemical compounds. For example, in the case of the aqueous solutions of inorganic bases (for example potash solution in the Benfield process) or organic bases (for example alkanolamines) used as chemical solvents most frequently on an industrial scale, salts are formed when sour gases are dissolved. The solvent can be regenerated by heating or stripping, the sour gas salts being thermally decomposed and/or stripped off by steam. After the regeneration process the solvent can be reused. Preferred alkanolamines used in the removal of sour gas impurities from hydrocarbon gas streams include monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), diisopropylamine (DIPA), aminoethoxyethanol (AEE) and methyldiethanolamine (MDEA).
Primary and secondary alkanolamines are suitable in particular for gas scrubbers in which the purified gas must have a very low CO2 content (for example 10 ppmv CO2). The nitrogen of the primary and secondary alkanolamines reacts directly with carbon dioxide, forming soluble carbamate. In the aqueous amine solution the carbamate is in a characteristic equilibrium with bicarbonate. To regenerate the amine solution, in industrial use, a two-stage regeneration process is frequently used, the loaded solvent firstly being expanded in one or more flash columns so that some of the absorbed CO2 vaporizes from the solution. Residual carbon dioxide and, if appropriate, other absorbed sour gases are then removed by stripping with steam. Solvents which comprise primary and secondary alkanolamines, however, require a greater amount of steam to decompose the carbamate than tertiary amines and correspondingly a great deal of heat energy; therefore, tertiary amines are frequently used.
European patent application EP-A 0 322 924 discloses using an aqueous amine solution which comprises tertiary alkanolamines, in particular MDEA, for deacidifying gas streams. In contrast to primary and secondary alkanolamines, tertiary alkanolamines do not react directly with carbon dioxide, since the amine is completely substituted. Rather, carbon dioxide is reacted with the tertiary alkanolamine and water to form bicarbonate in a reaction having a low reaction rate. Since no direct bond is formed between tertiary alkanolamines and carbon dioxide, the amine solution can be economically regenerated. In many cases, flash regeneration involving one or more expansion steps is sufficient here. An optional additional thermal regeneration requires significantly less energy than in the case of solutions of primary or secondary alkanolamines. Tertiary amines are suitable, in particular, for selective removal of H2S from gas mixtures which comprise H2S and CO2.
However, a disadvantage of the use of tertiary alkanolamine solutions is that, because of the low reaction rate of the carbon dioxide, the scrubbing process must be carried out with a very high residence time. The absorption and regeneration columns required are therefore very high, compared with systems in which either primary or secondary alkanolamines are used. Attempts have therefore been made to increase the absorption rate of carbon dioxide in aqueous solutions of tertiary alkanolamines by adding other compounds which are called activators or promoters.
German patent application DE-A-1 542 415 proposed increasing the activity both of physical solvents and of chemical solvents by adding monoalkylalkanolamines or morpholine and its derivatives. EP-A-0 160 203 mentions monoethanolamine as activator. German patent application DE-A-1 904 428 describes the addition of monomethylethanolamine (MMEA) as an accelerator to improve the absorption properties of an MDEA solution.
U.S. Pat. No. 4,336,233 describes one of the currently most effective scrubbing liquids for removing CO2 and H2S from a gas stream. This is an aqueous solution of methyldiethanolamine (MDEA) and piperazine as absorption accelerator or activator (aMDEA®, BASF AG, Ludwigshafen). The scrubbing liquid described there comprises from 1.5 to 4.5 mol/l of methyldiethanolamine (MDEA) and 0.05 to 0.8 mol/l, preferably up to 0.4 mol/l, of piperazine. Removal of CO2 and H2S with the use of MDEA is further described in more detail in the following patents of the applicant: U.S. Pat. Nos. 4,551,158; 4,553,984; 4,537,753; 4,999,031; CA 1 291 321 and CA 1 295 810.
International patent application WO 89/11327 discloses an absorption or scrubbing liquid which consists of an aqueous amine solution which comprises tertiary amines and small amounts of polyamines, for example aliphatic diamines as activator.